: The performance of feasibility tests is crucial in many applications. When using feasibility tests online only a limited amount of analysis time is available. Run-time efficiency is also needed for testing the feasibility of many different task sets, which is the case in system synthesis tools. We propose a fast uni-processor feasibility test using static priorities. The idea is to use approximation with a variable error to achieve a high performance exact test. It generally outperforms the existing tests which we show using a large number of random task sets. Key words: Real-Time Systems, Fixed-Priority Scheduling, Feasibility Analysis, FPTAS.